The present invention relates generally to reflective articles. The invention has particular application to retroreflective sheeting.
Retroreflective materials are configured to receive light rays impinging upon a viewing surface and to alter the rays so that they are reflected back toward their sources. Retroreflective material is generally used to enhance the low-light visibility of an article to which the retroreflective material is attached. Such material is used in a variety of applications ranging from traffic signs to bicycle reflectors. By enhancing low-light visibility, retroreflective materials enhance safety, provide decoration, and increase conspicuity in general.
Two known types of retroreflective material include microsphere-based sheeting and cube corner sheeting. Microsphere-based sheeting, sometimes referred to as “beaded” sheeting, employs a multitude of microspheres typically at least partially imbedded in a binder layer and having associated specular or diffuse reflecting materials (e.g., pigment particles, metal flakes, vapor coats) to retroreflect incident light. In general, however, such sheeting has a lower retroreflective efficiency than cube corner sheeting.
Cube corner retroreflective sheeting comprises a body portion typically having a substantially planar viewing surface and a structured surface comprising a plurality of cube corner elements such as cube corner projections and cube corner cavities. Each cube corner element comprises three approximately mutually perpendicular optical faces that intersect at a cube apex or, where the cube apex is truncated, that otherwise converge at an uppermost portion. It is known to treat the structured surface with a specularly reflective coating to improve performance at high entrance angles. An example of this is vapor-coated retroreflective sheeting.
Cube corner sheeting typically has a much higher retroreflectance than beaded sheeting, where retroreflectivity or brightness (these terms are used interchangeably herein) is expressed in units of candelas per lux per square meter. However, certain graphics applications require not only certain levels of retroreflectance but also particular levels of daytime “whiteness.” The whiteness of an object is sometimes described in terms of the second of the tristimulus coordinates (X,Y,Z) for the object, and thus is referred to as “cap-Y.” The cap-Y scale ranges from 0 for a perfectly black object to 100 for a perfectly white object.